The invention relates to a process for obtaining xenon and krypton from a liquid oxygen (LOX) charge, with xenon (Xe), krypton (Kr) and hydrocarbons (CxHy) in a low concentration and approximately 99 mol percent oxygen (O2). The LOX charge is supplied to a 1st column, the oxygen of the LOX charge being largely stripped out by means of an inert gas and being obtained in the head gas, while the inert gas with little O2 and almost the total amount of CxHy, Kr and Xe is withdrawn in a liquid state from the bottom of the 1st column.
The stripping-off of oxygen by means of argon in a stripping column is known from M. Streich, P. Daimler, xe2x80x9cObtaining Precious Gases in Air and Ammonia Systemsxe2x80x9d, Linde Berichte aus Technik und Wissenschaft 37 (1975). As a result of this measure, during the upgrading of the hydrocarbons, together with the krypton and xenon, the oxygen content remains under the ignition limit of a reaction of the hydrocarbons with oxygen.
It is an object of U.S. Patent Document U.S. Pat. No. 4,401,448 to replace the oxygen in such a stripping column by nitrogen.
Subsequently, in both processes, at least a large portion of the hydrocarbons are adsorptively or catalytically removed with a subsequent adsorption of the reaction products water and carbon monoxide. The system necessary for this purpose requires high expenditures. A continuous operation can be achieved only by the alternative charging and regenerating of at least two adsorbers, in which case the process flow with the CxHy must be switched at intervals to the respectively regenerated adsorber.
From German Patent Document DE-OS 16 67 639 (see, among others, the claims and the figures with the pertaining explanation in the text), a process is also known which uses a liquid oxygen charge of an air separation system (LZA), in which the oxygen is largely replaced by the inert gas argon. However, the hydrocarbons are removed corresponding to the state of the art known from U.S. Patent Document U.S. Pat No. 4,401,448 and M. Streich by means of adsorption and/or catalytic combustion.
It is therefore an object of the invention to provide a process which is simple and which can be operated without switching the process flow.
According to the invention, this object is achieved by means of a process having the characteristics of claim 1. Further developments of the invention are the object of subclaims.
It is a characteristic of the invention that the withdrawn liquid is supplied to a second column without any previous catalytic and/or adsorptive removal of CxHy. A Kr fraction is obtained as head gas of the 2nd column and an Xe fraction is withdrawn from the bottom of the 2nd column. The Xe fraction from the bottom of the 2nd column is fed in a center part between the head and the bottom of a 3rd column and a pure Xe product is withdrawn at the head of the 3rd column.
In addition to the Kr, all constituents which boil more easily than Xe, particularly methane, are contained in the Kr fraction. If this Kr fraction is abandoned and only the Xe fraction, which is by one order of magnitude smaller, is processed for obtaining the economically more significant xenon, the process-related expenditures are considerably reduced. In the case of smaller systems, this justifies the elimination of possibly obtaining Kr from the Kr fraction. In an embodiment of the process described below, however, the Kr can also be obtained in a simple manner as an additional product.
The inert gas can be fed above the bottom of the 1st column.
The inert gas can be obtained from an air separation system (LZA) at the site and contain mainly nitrogen and/or argon.
As a result of an air separation system at the site, a required making available of the inert gas is therefore eliminated. If the inert gas contains argon, the argon can be recovered in the air separation system from the head gas of the first column.
The LOX charge can be taken from an air separation system existing at the site. It is less advantageous to transport the LOX charge for obtaining the Xe.
The LOX charge can be fed at the head or several floors below the head of the 1st column.
As required, the pressure of the LOX charge can be adapted to the pressure at the head of the 1st column. Thus, an operating pressure of the first column can be adjusted which is optimal as a function of the inert gas used for the stripping.
The bottom of the 1st column can be heated by an indirect heat exchange. For the heating, an electric heater or a process flow of the air separation system at the site can be used. When an electric heater is used, the process will be independent of the operation of an air separation system; otherwise, electricity costs are saved.
The head of the 1st column can advantageously be cooled by a direct or indirect heat exchange.
In the event of a use of nitrogen as inert gas, liquid nitrogen can be used for cooling the head of the first column. Liquid nitrogen can also easily be made available at sites without an air separation system.
Preferably after a pressure increase, the withdrawn liquid from the 1st column can advantageously be fed several floors below a head condenser of the 2nd column. The pressure increase may be useful in the case of unfavorable hydrocarbon admixtures for avoiding a precipitation of solid matter.
The Xe fraction from the bottom of the 2nd column can be fed in a central part between the head and the bottom of a 3rd column, and a pure Xe product can be withdrawn at the head of the 3rd column. With approximately 99.999 mol percent xenon, this product can at least partially be marketed directly or may possibly at a different site be used for obtaining purest Xe.
The Kr fraction can be fed from the head of the 2nd column into a central part between the head and the bottom of a 4th column, and a pure Kr product can be withdrawn from the bottom of the 4th column. Analogous to the pure Xe product, the pure Kr product with 99,999 mol % krypton can be marketed directly and/or be used for obtaining purest Kr.
The head of the 2nd and/or 3rd and/or 4th column can be cooled and the bottom of the 2nd and/or 3rd and/or 4th column can in each case be heated by an indirect heat exchange by means of a fluid or by means of an electric heater.
The process according to the invention can be used in a device for obtaining Xe and/or Kr at an air separation system.
The device for obtaining Xe and/or Kr can be arranged in a transportable container. This permits, on the one hand, a particularly simple mounting on an air separation system and, on the other hand, such a device can also be used as a mobile device for several air separation systems. For this purpose, it is only necessary to intermediately store LOX charge from the air separation system and process it, when the opportunity arises, in the mobile device. This saves extensive transports of LOX charge, which contains the valuable xenon substance only at approximately 400 mol ppm.
The invention will be explained in detail by means of an embodiment in a FIGURE.